Power you can generate. Water you can store. Wastewater has to go somewhere an authority accepts, and that is what determines whether the plot works.
Grey water is the real constraint
Shower, basin and kitchen produce the daily volume, and it is grey water that most owners underestimate. A composting toilet removes the black water question and leaves grey water entirely unresolved.
The options are a connection to the sewer, a package treatment plant, or a reed bed and soakaway where the ground and the authority both permit it. All three are assessed before the building consent, not after.
A mains connection is worth measuring first
Utilities price by the metre and the trench is the expensive part. Fifty metres to the boundary is a different project from four hundred, and the answer changes which of the options above is even worth pricing.
Ask for a quotation as soon as the plot is identified. It is free, it takes a fortnight, and it is the number the rest of the budget hangs off.
Tanks: size on days, not litres
Two people use roughly 100 to 200 litres a day with normal fixtures and considerably less with an aerated shower head and a low-flush toilet. Size the fresh tank on how many days you want between fills and the grey tank to match, or you are emptying one while the other is still half full.
Put both where they can be drained and inspected. A tank that freezes because it sits in an unheated void is a predictable failure and an avoidable one.
Rainwater is easy for some uses and hard for others
Collected roof water is straightforward for toilet flushing, laundry and the garden. For drinking it needs filtration and disinfection that can be demonstrated, plus a service routine — it is an installation, not a purchase.
Roof area on a tiny house is small. Run the arithmetic for your rainfall before committing: 30 square metres of roof in a dry summer month is less water than people expect.
Freeze protection is a winter design item
Pipes in unheated voids, the outside tap and the supply at the chassis are where a freeze starts. Trace heating and a drain-down valve solve it, and for a let unit the drain-down has to be a written procedure with a named person.
Design it in. Retrofitting trace heating to a closed floor cassette is not a job anybody enjoys.